1. Field of the Invention
The present invention relates to a high-frequency switch for changing between signal paths in a high-frequency circuit of, for example, a mobile cellular telephone. More particularly, the invention relates to a high-frequency switch having four ports and using transmission lines.
2. Description of the Related Art
In some mobile cellular telephones, either two antennas or one antenna and an external connecting terminal are for the common use in the transmitting circuit and the receiving circuit. In this type of telephone constructed as described above, a switch circuit shown in FIG. 7, for example, is conventionally used.
A switch circuit generally designated by 51 is constructed by connecting two three-port switches 52 and 53 to each other. The switch 52 has first, second and third ports P21, P22 and P23, while the switch 53 has first, second and third ports P31, P32 and P33. The second port P22 of the switch 52 is coupled to an antenna ANT, and the third port P23 is used as an external connecting terminal EXT. The third port P23 may, for example, receive a predetermined signal from the terminal EXT in order to measure the electrical characteristics of the receiving circuit of a mobile cellular telephone having this built-in switch circuit 51. Further, a second antenna may be connected to the third port P23. Namely, in a vehicle-mounted cellular telephone, a vehicle-mounted antenna may be connected to this circuit 51 as the second antenna.
In the switch 52, the first port P21 is adapted to be connected either to the second port P22 or to the third port P23. The first port P21 of the switch 52 is coupled to the first port P31 of the switch 53. The first port P31 of the switch 53 is adapted to be connected either to the second port P32 or to the third port P33. The second port P32 is connected to a transmitting circuit Tx, while the third port P33 is coupled to a receiving circuit Rx.
The switch circuit 51 constructed as described above can be used to connect either the antenna ANT or the external terminal EXT to the transmitting circuit Tx or the receiving circuit Rx.
As a component for providing each of the above-described three port switches 52 and 53, a high-frequency switch using diodes illustrated in FIG. 8 is known. A high-frequency switch generally designated by 61 has first through third ports P61 through P63 corresponding to the first through third ports P21 through P23 and P31 through P33 of the afore-described respective switches 52 and 53. The first port P61 is connected to the cathode of a diode 65a through a capacitor 64. The anode of the diode 65a is coupled to the second port-P62 through a capacitor 66a. Further, a transmission line 67a is connected at one end to the node between the anode of the diode 65a and the capacitor 66a. The transmission line 67a is formed of a stripline, a microstrip line, a coplanar guide line, or a high-impedance transmission line, all of the lines having a length slightly shorter than .lambda./4 when the wavelength of a high-frequency signal sent to this switch 61 is represented by .lambda.. The transmission line 67a is grounded at the other end across a capacitor 68a. Moreover, a resistor 69a is connected at one end to the node between the transmission line 67a and the capacitor 68a and at the other end to a control voltage terminal VC1. The first port P61 is coupled across the capacitor 64 to a transmission line 71 which is constructed in a manner similar to the transmission line 67a. The other end of the transmission line 71 is grounded.
Additionally, connected to the first port P61 is the cathode of the diode 65b through the capacitor 64. The anode of the diode 65b is coupled to the third port P63 through a capacitor 66b. In a manner similar to the diode 65a, a series circuit of a transmission line 67b and a capacitor 68b is connected between the diode 65b and a ground potential. Also, a resistor 69b is coupled at one end to the node between the transmission line 69b and the capacitor 68b and at the other end to a control voltage terminal VC2.
In the high-frequency switch 61 constructed as described above, different control voltages can be applied to the control voltage terminals VC1 and VC2 so as to connect the first port P61 either to the second port P62 or to the third port P63. For example, by application of a positive control voltage to the control voltage terminal VC1 and a negative control voltage to the control voltage terminal VC2, a forward bias voltage can be applied to the diode 65a, while a reverse bias voltage can be applied to the diode 65b. Namely, since the capacitors 66a, 68a, 64, 66b and 68b interrupt the flow of direct current, the control current supplied from the control voltage terminal VC1 flows into a circuit portion including the transmission line 67a, the diode 65a and the transmission line 71, thereby causing the diode 65a to be activated. In contrast, the diode 65b is rendered to be ineffective since a reverse bias voltage is applied to the diode 65b.
Further, when a high-frequency signal is supplied from the second port P62, the transmission line 67a constructed as described above can be grounded at one end in a high-frequency band, and the impedance viewed from the node between the anode of the diode 65a and the capacitor 66a to the series circuit formed of the transmission line 67a and the capacitor 68a is caused to become infinite due to the impedance inversion of the wavelength .lambda./4. Accordingly, the high-frequency signal fed from the second port P62 flows into the first port P61.
Conversely, by application of a negative control voltage to the control voltage terminal VC1 and a positive control voltage to the control voltage terminal VC2, a reverse bias voltage can be applied to the diode 65a, while a forward bias voltage can be applied to the diode 65b. This causes the diode 65a to be rendered ineffective and the diode 65b to be activated. Thus, a signal flows between the first port P61 and the third port P63 rather than between the second port P62 and the first port P61. The impedance viewed from the node between the anode of the diode 65b and the capacitor 66b to the series circuit formed of the transmission line 67b and the capacitor 68b is also caused to become infinite, thereby preventing a high-frequency signal from flowing into the transmission line 67b.
The transmission lines 67a and 67b constitute current paths which allow a control current to flow into the diodes 65a and 65b, respectively. The lines 67a and 67b also function to enhance the impedance viewed from the node between the anode of the diode 65a and the capacitor 66a and the node between the anode of the diode 65b and the capacitor 66b to the transmission lines 67a and 67b and to reduce insertion losses and reflection losses.
As discussed above, in the high-frequency switch 61, by the application of a positive control voltage to one of the control voltage terminals VC1 and VC2 and a negative control voltage to the other terminal VC1 or VC2, the first port P61 can be connected either to the second port P62 or to the third port P63.
The switch circuit 51 shown in FIG. 7 can be constructed by using two of the afore-described high-frequency switches 61 as the switches 52 and 53. That is, the switch circuit 51 can be constructed by connecting the first ports of the three-port high-frequency switches 52 and 53 to each other.
Thus, by the use of the two switches 52 and 53 for this switch circuit 51, a high-frequency signal inevitably passes through the two switches 52 and 53. For example, a transmission output supplied from the transmitting end Tx passes through the two switches 52 and 53 until it reaches the antenna ANT. Similarly, a high-frequency signal input from the antenna ANT passes through the switches 52 and 53 and is output to the receiving end Rx. This inevitably increases insertion losses. Accordingly, the transmission output is required to be increased in the transmitting circuit Tx, and a decrease in gain is incurred in the receiving circuit Rx.
Additionally, since the switch circuit 51 is formed by using the two high-frequency switches 61 as the switches 52 and 53, it is necessary to apply control voltages to the two control voltage terminals of the switches 52 and 53. This entails the provision of two power sources for supplying control voltages. As a consequence, a complicated wiring pattern for providing the power sources must be formed on a circuit board.